In hydrodynamic bearings, a rotating object such as a shaft is supported by a stationary bearing pad via a pressurized fluid such as oil, air or water. Hydrodynamic bearings take advantage of the fact that when the rotating object moves, it does not slide along the top of the fluid. Instead the fluid in contact with the rotating object adheres tightly to the rotating object, and motion is accompanied by slip or shear between the fluid particles through the entire height of the fluid film.
Thus, if the rotating object and the contacting layer of fluid move at a velocity which is known, the velocity at intermediate heights of the fluid thickness decreases at a known rate until the fluid in contact with the stationary bearing pad adheres to the bearing pad and is motionless. When, by virtue of the load resulting from its support of the rotating object, the bearing pad is deflected at a small angle to the rotating member, the fluid will be drawn into the wedge-shaped opening, and sufficient pressure will be generated in the fluid film to support the load. This fact is utilized in thrust bearings for hydraulic turbines and propeller shafts of ships as well as in the conventional hydrodynamic journal bearing.
Both thrust bearings and radial or journal bearings normally are characterized by shaft supporting pads spaced about an axis. The axis about which the pads are spaced generally corresponds to the longitudinal axis of the shaft to be supported for both thrust and journal bearings. This axis may be termed the major axis.
In a large steam turbine, several stages of blades are mounted on the steam turbine shaft and axially spaced-apart along the shaft to form a complete rotor. Each set of blades or airfoils, or each turbine stage, changes the enthalpy of the steam passing axially through the turbine which causes the rotor to rotate. The force of the steam admitted into the turbine affects the rotor. As is well known in the art, the direction and magnitude of this force is influenced by the particular control mode of operation for the turbine, i.e., full arc steam admission mode or partial arc mode. Hence, although the rotor primarily rotates about its axis, the turbine shaft also experiences both horizontal and vertical movements due to these forces.
Commonly a plurality of bearings are located at various axial locations along the shaft. Some bearings of a steam turbine include several pads which space the rotatable shaft away from the bearing casing. These bearings are normally lubricated with oil and some of this oil is distributed between each pad face and the shaft's surface. In operation, the oil in the interstice between the pad face and the shaft surface hydrodynamically lifts the shaft off the face of the pad. The amount of lift developed in the bearing determines the stiffness of the bearing to horizontal and vertical forces acting upon the shaft. In this manner, the bearing dampens the horizontal and/or vertical movements of the shaft, as well as, rotatably supports the shaft without placing large frictional forces thereon which inhibit the rotation of the shaft. The frictional forces inherent within the bearing, and hence power losses, are minimized by the oil film in the interstice defined by the rotating shaft surface and the face of the pad. Additionally, the oil film cools the pad face, which is heated by friction, thereby protecting the integrity of the bearing.
Due to the great weight of the turbine carried by the shaft in combination with the speed of rotation of the shaft, a bearing which loses this oil film in one or all of its interstices deteriorates rapidly because the shaft surface wipes the pad face and, consequently, the shaft and/or the pad face may be scored. The resulting inefficiency of a wiped bearing is well known in the art. Additionally, when the shaft surface does come in wiping contact with the pad face, great frictional forces are generated by that contact which affects the immediate performance of the steam turbine.
Since horizontal and vertical damping of the turbine shaft motion is an important function of the bearing, three pad bearings have been developed. The three pad bearing lessens the total amount of pad face area which interacts with the shaft surface, thereby lowering the total viscous shear of the oil, and hence, lowering the total frictional forces and power losses developed within the bearing. However, the minimization of the shaft surface/pad face interface introduces arcuate spaces between each pad, i.e., the space defined by the trailing edge of the preceding pad, the leading edge of the next or succeeding pad, the shaft surface and the radially inner surface of the bearing casing.
Since the lubricating oil cools the pad face, as well as provides support for the shaft, a continuous stream of oil normally flows through the interstice between each pad face and the adjacent shaft surface. The oil is ejected proximate the trailing edge of each pad. The ejected oil churns within the arcuate space and such churning is believed to cause some power loss in the bearing.
Such bearings are described for example in the U.S. Pat. No. 4,497,587.
In modern steam turbines the load of the rotor can exceed 200 tons. In spite of the great load the bearing are typically manufactured to very small tolerances.
It can be seen as an object of the present invention to change the stiffness of the bearing and hence increase the rigidity of the bearing and its support structure.